A numerical model of organic devices，which includes charge injection, transport, space charge effect and trap influence，was discussed in this paper. Both thermionic emission over the barrier and tunneling through the barrier are considered in charge injection into the bulk. The result is in good agreement with the experimental data. Several parameters have been simulated to study the change of J-V characteristics. The current decreases with the increases of the length of device. The current density becomes higher when the hole injecting barrier is smaller. However, when the electronic injecting barrier decreases from 1.7 eV to 0.5 eV, the current density becomes smaller. This is because the electronic mobility is too small, and the electronic injecting current is negligible compared with the hole current. When the electronic injecting barrier is smaller, the built-in potential becomes bigger, under the same applied voltage, the field diminishes, and the hole current becomes smaller. When the barrier is reduced to 0.1 eV, the current density increases. This is because the electronic mobility is too small, and electrons accumulate near the cathode, the electric field around the anode increases and the current increases.